Distillation process



17 Water March so, 1943. w. w. KRAFT 2,315,122

'msnm-JuxowrnocEss Filed Feb. 28, 1958 v I INVENTOR W/zeaton 'BY V AORNE Y Patented Mar. 30, 1943 DISTILLATION PROCESS Whcaton W. Kraft, NewYork, N. Y., assignor to The Lummus Company, New York, N. Y., acorporation of Delaware Application February 28, 1938, Serial No.193,168

1 Claim.

This invention relates to improvements in petroleum refining processesand more particularly to an improved arrangement for the condensing andcooling of the lowest boiling range product topped crude bottoms productremoved at It. In this case; however, the'topping unit H3 is preferablyoperated underpressure which may be in the neighborhood of fifty poundsper square inch.

from a topping unit. 5 The overhead vapors are discharged into the heatIt is well known that in the operation of topexchanger I5 which is watercooled by Water ping units, at substantially atmospheric pressure,entering at l 6 and discharging at I]. the amount of uncondensed gaseousmaterial The heat exchanger [5 is in the nature of a reaches extremelyhigh proportions especially partial condenser and cooler, but has suchheat when operating on light or wild crude oils. l removal capacity thatadequate reflux liquid may These amounts may range as high as and are beobtained. The heat transfer surface is much frequently in the range of3-5%, depending upon smaller than usual in this apparatus for even theavailability of cooling water at a suitable though it handles the entireoverhead, 'the mean temperature and other operating conditions.temperature 'difierence between the relatively With a unit having acapacity for 25,000 barrels hot vapors and the cooling water is quitelarge Der day of crude oil, it will be appreciated that due to thepressure operation and the fact that the amount of such uncondensedmaterial is very t efi s I101; Cooled to end Product p substantial and,even more objectionable, it reature. quires such a large extent ofapparatus to handle he condensed l q d and Uncondensed VdDOIS it, thatit becomes a major problem in petroleum from the first heat exchanger orpartial conrefining. denser I 5 are then collected in the receiver I8,

The principal object of the present invention is the liquid level ofwhich is maintained at a deto provide an improved method for reducing tosired level by the control l9.' The reflux liquid is negligibleproportions, the uncondensed gaseous removed at 20 and p p into thereflllX line 2! material loss from a topping operation and s0 to the topof the fractionating column It. 'Atemabsorbing the gas that it can bereadily handled pera ure' n rol i p vi e at 22 to r l the in combinationwith the liquid whereby extensive amount of reflux in accordance withthe Vapor equipment is not required. temperature discharging overhead ati2. The Another object of the invention is to provide reflux, which inusual practice, is several times an improved process for condensing andsub-coolthe volume of final product discharged at 32, is ing theoverhead from a topping unit so that the not as cool as is customary butas the reflux is apparatus may be of low cost and will require purely aheat removal process, the higher rel'elatively smaller amounts ofCooling water at turn temperature is not objectionable for moreavailable temperatures with a maximum efliciency eflux m y be etu demuch higher maxiof cooling. mum temperature of the overhead, more than Amore specific object of the invention i t balances this higher minimumtemperature with operate a topping unit under such pressure, and theresulting higher mean temperature difference. to condense only so muchof the vapors, and to e u oo de ed se fro e ce ver l8 sub-cool only theend product, that higher heat so ove head at 4 unde p s e control25,.With removal efliciency is obtained by a higher mean 40 e Vapor e 24jo d With p t line S0 temperature difference. and the uncondensed h t thvap r nd iqu pr t e com ined gas is reduced to a negligible quantity, ndi n and introduced into the final condenser or subhandled as absorbed inthe liquid end product. Cooler nd o r 6 hr h line 2 Th fin Furtherobjects and advantages of t invoncooler 26 is also water cooled With theinlet at tion will appear from the following description 23 and thedischarge t of a preferred form of embodiment thereof taken Pressure t l25 is OOIlVenieIltlY inserted in in connection with the attached drawingwhich line 24 t ta the pre u o ac o at illustrates diagrammatically t tpart of a pgcolumn l0 and partial condenser l5 substantially troleumrefining plant which specifically perconstant and equal. Althoughcondenser 26 is tains to the condensation and sub-cooling of the a d ata p e Which y be the Same lightest fraction from a t p i unit lower thanthe pressure on the rest of the system, The toppin u it to in accordancet my nit is desirable to utilize a pressure control as at vention, is ofany suitable type with the lightest 5 to p v t flu a n t p ure n tfraction discharged as Overhead Vapor at '2, the discharge end of thesystem from affecting the heated crude oil being introduced at l3, andoperation in the fractionating column and the partial condenser. It willbe understood, of course, that this pressure control device may be soplaced as to maintain both of the condensing stages and thefractionating column under the same pressure.

The higher mean temperature difference in the partial condenser I 5requires a much smaller amount of heat exchange surface than thesubcooler 26 per unitof heat removed; This is due to the fact that thecooling water entering the sub-cooler 26 is not very much below thenecessary temperature of the sub-cooled end product. By virtue of notsub-cooling reflux material, it can be seen that a very great decreasein total heat exchange surface results. This approximately balances thedifference in volumes of materials handled.

The resulting end product, which may be a light gasoline is dischargedat 32, and is foundto con,-

tain less than a negligible quantity of uncondensed gas, and this may bedirectly run to a suit-' able stabilizing unit. No separation of gasfrom liquid is required, and the end product'has such a relatively smallproportion of gas, it may be handled as a liquid without difiiculty. Noexcessively large equipment isrequired under the cir cumstances. V

As an'example of operating conditions under which this process operates,I have used pressures within the fractionating column II] in theneighborhood of fifty pounds per square inch with equivalenttemperatures of approximately 290" F. This compares with normalatmospheric pressure operating temperatures of 175 F. The reflux thenreturns to the fractionating column at about 140 to 150 F. instead ofapproximately 105 F.

With cooling water at a minimum temperature of 95 F., it will be seenthat, in the partial cooler I5, a very large amount of heat may beremoved with a relatively high mean temperature differ ence which wouldnot exist if allofthe overhead had to be cooled to 105 F.; cooling theend product onlyto this'lowtemperature thus makes a direct saving inheat exchange surface.

reducing the uncondensed gas from 5% to 0.5% or less, approximatelyninety per cent of the gas handling equipment can be eliminated. Thissaves on gas separators, gas conduits, gas holders, and gas compressors,all of which are extremely costly. A typical multiple pass shell andtube heat exchanger for the final cooler 26 is most satisfactory for therequired absorption of the liquid and vapor during the sub-cooling.

While I have shown a preferred form of embodiment of my invetnion, I amaware that modifications may be made thereto and I, therefore, desireabroad interpretation of my invention The pressure operation alsopermits direct handling of the end product as a liquid and by within thescope and spirit of the disclosure herein, and of the claim appendedhereinafter.

What I claim is: The method of condensing hydrocarbon vapors and gasesobtained as an overhead from the fractionation of hydrocarbons undersuperatmospheric pressure in which the hydrocarbons undergoingfractionation are crude petroleum, and the fractionation treatment is atopping operation, which comprises cooling the vapors and gases toproduce a partial condensation thereof, separating the resultingcondensate from the uncondensed vapors and gases, returning a sufllcientportion of said condensate as reflux for said fractionation, combiningthe uncondensed vapors and gases with the remainder of said condensate,cooling the mixture to a temperature lower than that of the partialcondensation to effect a complete condensation of the vapors and anabsorption of the gases in the condensate so that the final productcomprises a liquid containing less than of uncondensed material, andcontrolling the pressure at which the uncondensed vapors and gases areseparated from the partial condensate in the partial condensation stageindependently of the pressure on the final condensation and coolingstage, the pressure on the partial condensation stage beingsubstantially equal to the pressure of fractionation and higher than thepressure on the final condensation and cooling stage. A

. WHEATON W. KRAFT.

